Transmission system



Patented July 2, 1929.

UNITED STATES P ATE NT 0F i E FRANK E. FIELD, OF SOMERVILLE, NEW JERSEY, ASSIGNOR TO WESTERN 'ELEOTRIG' COMPANY, INCORPORATED, OF N EW YORK, N. Y., .A CORPORATION OF' NEW YORK.

I TRANSMISSION SYSTEM.

A pneauon filed March 31,192 Serial No 98,758.

This invention relates to transmission systems and particularly to transmission :systems employing transformers.

An object of the invention isto improve the transmission characteristics of electric circuits.

A related object of the invention is to repeat a wide range of frequencies with sub-.

sistive impedance to the capacitive imped-.

ance represented, for. example, by the. gridfilament circuit of a space discharge tube am plifier'.

In systems ofthis type it is important that the amplifier have a substantially flat gain characteristic over a wide frequency range in order to faithfully: transmit or reproduce the essential music or speech frequencies. It is well known, however, that the inductance of an input transformer of an amplifier will resonate with the distributed capacity of the transformer and the capacity of the amplifier, producing a peak in the transmission characteristic curve. As a result, the gain of the amplifier at and near the resonance frequency is greater than the'gain at the other transmitted frequencies, and in some cases may be so pronounced that the amplifier will sing orvhowl at the resonance frequency. a

This difliculty is largely overcome, in accordance with this invention, by utilizing the eddy current loss in a transformer to give the transformer a fiat transmission characteristic over a wide frequency range. According to a feature of the invention, the magnetic structure of the transformer is so arranged that the effective inductance of the transformer is high at very low frequencies and there is very little eddy current loss, resulting in a high gain. As the frequency rises, however, the eddy current loss increases and the effective inductance of. the trans former, decreases. By thus materially decreasing the inductance of the resonant cir:

cuit at the higher frequencies,it is possible, Within certain limits, to raise the resonance to such a predetermined point that it will have no effect on thetransmission efficiency of the transformer within range to be transmitted r The various features and advantagesof theinvention will be described in detail in connection with the accompanying drawing, in which: I

' Fig. 1 is a diagrammatic illustration of an amplifier circuit embodying the invention;

Fig. 2 is a perspective view of a transformer having a magnetic core constructed in accordance with the invention; f

Fig. 3 shows curves illustrating the drop in effective permeability of. various thicknesses of core material with increases in frequency; and i V the. frequency Fig. 4 shows curves illustrating the characteristics of several different transformer circuits. i

In Fig. 1, an incoming line ,5 is connected to an outgoing line 6 by means of a two-stage amplifier comprising space discharge tubes 7 and 8 connected in tandem. Direct current is supplied to the cathodesof these tubes from the battery 9, and space current is suppliedto theanodes from battery 10. Gridpolarizing batteries 11 and 12 are associated witlrthe tubes 7 and 8,'respectively. The line ,5.i s coupled to the input circuit of tube 7 by means of an input transformer 13. The tubes, 7 and 8 are coupled together by an interstage transformer 14, and the output circuit of the tube 8 is coupled to the outgoing line 6 by means of an output transformer 15. ,The transformers 13, 14 and 15 comprise primary and secondary transmit a wide band of frequencies.

It is Well, known-thatthe effective permeability of magnetic materials varies with thefrequency of the magnetizing force to which it is subjected. This is due'to tllfiflc tion of the eddy currents in the material which cause a diminution of the effective flux and hence a drop in the effective permeabih ity. netic circuits of transformers 13 and l lrare According to this invention the magwindings and are designed to.

of the type illustrated in Fig.2. This trans-- former comprises two similar E-shaped magnetie 'core sections 26 and 27 Wound in the usual manner with. superimposed primary and secondary windings indicated generally at 28. Any suitable type of clamping device maybe employed to hold the assembled structureintact. f

suitable. magnetic material for the core sections 26ahd27is a nickel-iron alloy known perinallo'y, which, when subjected to the proper heat treatment-,h'as an extremely high permeability when operated at low flux densitie'sf This material is -a composition consisting chiefly of nickel and iron containing from 30% to 90% nickel, depending upon the particular."characteris ics desired. Particularly good results have been obtained with a coin-position consisting :of approximately 78- nickel and' 21%% iron. The characteristics and method er preparing these alloys are disclosed in United -States Patent 1,580,884granted to G. W. Elmen, dated June 7 1, 1926, and are also described in some detail in a paper by H. D. Arnold and G. NV. Elmen entitled Permall'oy, publishedin the May 1923 issue of thedournal of the Franklin Institute.

It is a property'of permalloy thatits permeabili'ty at very low magnetizing forces is extraordinarily high, values of the order of 6,000 for zero magnetizing force being easily obtainable, whereas the corresponding value 1 for the best gradeof iron is only about 300.

The value of the permeability at zero force is obtained by determining a series of values for exceedingly low' forces, say of the order of .01 'gauss to .'5' "ga'uss. The results plot linearly and may be extrapolated back to the Value for H O. v

The eddy current loss in permalloy core transformers increases with the frequency of the transmitted waves much more rapidly than in transformers employing other types of magnetic cores, such as iron or silicon steel. The eddy-'currentloss, however, may be controlled by constructing the permalloy core in a plurality of sections or laminations of predetermined thickness, the several sections or laminationsbeing insulated from each other when C mPai-atively-s-niaIl eddy current losses are 'desired In order to provide a core'having an eixtremely'hi gh eddy current loss fac tor, the core may consist of uni'nsulated laminations, since the oxide coating produced during the heat treatment of permalloy has a low resistance, or it may evenconsist of a solid core, depending upon the particular characteristics desired.

In describing the preferred method of deis given e +26% e 2 cos 201,,

in which, I

m=apparent flux density B =fiux density produced by, external 6 e 2 cos 201 .1nagnetomotive force Z the thickness of the laminations f=frequency i=permeability at Zero frequency A electrical conductivity.

Thisformula gives the relation between the flux density at zero frequency. and any other frequency. Since the flux density is proportional to the permeability this formula can be used to determine the permeability of magneticmaterials at various frequencies.

Assun'iing that silicon. steel. has a normal zero frequency permeability of 320 and a-conductivi-ty of 2 X the permeability .of 14 mil laminati'ons at various frequencies may be computed. This data is illustrated graphically by the curve of Fig. 3 in whichthe ordinates represent the percentagepermeability and the abscissae represent kilocycles. The curves 31, 32, 33 and 3a illustrate the permeability at various frequencies Of'PEllllalloy laminations comprising 78 7 nickel and 21 0 iron, which are 6 mils, 14 mils, inch-and inch thick, respectively. The perm-alloy employed in all of the-samples is assumed to have a zero frequency pern'ieability of 10,000 and a conductivity of 6.24 'X 10*. It will be seenfrom curves 31',- '32, 33 and 34 that as the thickness of the permalloy laminations is increased, the drop in permeability becomes greater, the drop in the case ofpermalloy laminations inch in thickness, represented by curve 34 being about 94% at'500 q duced.

pacity shunted across the secondary terminals of the transformer. This parallel combination of L and Cwill resonate ata frequency obtained by the relation in which L is expressed in henries, C in farads,

and f incycles per second Knowing the in ductance of a transformer and the distributed capacity of the particular type of winding used, it is possible by means of this relation to calculate the resonance frequency of the transformer; Let it be assumed, for example, that an input transformer winding of a certain type has a distributed capacity of 20 micro-microfarads, and, when a silicon steel core made up. of Hamil laminations is inserted, has a total. inductance of .500 henries. The resonance frequency, calculated by means of the above formula, is approximately 1590 cycles. Curve. 30. of Fig. 3 shows that at this frequency, the permeability has not begun to drop and hence the transformer will actually resonate at 1590 cycles. 1

'lVhenthe silicon steel core is replaced with a per-malloy core comprising a plurality of 14 mil laminations, the total. inductance of the transformer will now be at least 5000 henries, and a calculation by means of the. above formula shows that the resonance pointwill be at 500 cycles assuming that the permeability remains constant. Actually, however, the permeability does not remain constant since, asshown by curve 32 of Fig. 3, at 500 cycles the permeability has been reduced to 36% of its original value, or 3600, which results in an inductance at that frequency of 1800 henries. This inductance would cause resonance at837 cycles if" the permeability remained constant, but at that frequency the permeability has been rcducedto28% of its original value, re-

sulting in an inductance of 1400 henries. This inductance wouldresonate with the capacity at 950 cycles were it not for the fact that at this point the permeabilityis still further re- Following this method it is found that the transformer would actually resonate at about 1,000 cycles. That is, the action of the eddy currents in the core has resulted in raising theresonance point from 500 cycles to 1,000 cycles. a a

Let it'now be assumedthat the 14 mil laminated per-malloy core is replacedby a core of the same material having laminations inch thick, as illustrated graphically by curve 34 of Fig.3. Bya series of calculations, it

decreases.

is'found that the, transformer would then resonate ata frequency of about 300cycles. In this case the action of the eddy. currents has resulted in a shift of the resonance point from 500 cycles to 4300 cycles.

.In a circuit such as that illustrated in Fig. 1 when transformers of fairly high ratio are employed, the maximum amplification is obtained at the frequency at which the inductance of the transformer resonates with the effective capacity across its secondary winding. This is true, for example, when the turnsratio of the transformer is greater than 5 to land when the impedance of the circuit to which the primary of the transformer is connected is more than 20,000 ohms. At frequencies above and below the-resonance frequency the amplification falls off rapidly. For lower turns ratio or lower impedances in the circuit connected tothe primary, the curve flattens out considerably. WVitha silicon steel core such as that described above the amplification characteristic will be'as' shown by the curve 40 of Fig. 4,in which the ordinates represent voltage amplification and the abscissae frequency in cycles per second. When 14 mil per-malloy laminations are employed the resonance point is lowered and the transmission characteristic curve is flattened out as shown by the curve 41. The lowering of the resonance point in this case is due to the muchhigher inductance of the windings. Curves 42 and 13 show the shape of the transmission characteristic when inch and. inch permalloy laminations are employed, respectively, the resonance point being'raised in each case because ofthe lowering of the inductance due to the action'of the eddy cur.- rents. As the resonance point is raised, the curve becomes flatter and the quality of the transmitted speech and music is improved. The capacity of the vacuum tube and the rest of the circuit connected directly tojthe secondary of the transformer has for clarity been neglected in this discussion. It will, however, add directly to the distributed capacify of the transformer and produce a correspondinglowering of the resonance frequency. I y g The input transformer 13 of, Fig. 1, for example, when constructed in accordance. with the invention has a high effective inductance and Very little eddy current loss at low frequencies. As the frequency rises, however, the eddy current loss increases and the effective inductance of thetransformer Since the capacity effectively in shunt, to the, secondary winding remains practically constant, the frequency at which i the inductance of the transformer resonates with theeffective shunt capacity is raised to a point where it has no. effect on the transmis sion efficiency of the transformer within the frequency rangevtransmitted. f

The invention is, of course, susceptible of various other modifications and adaptations not specifically referred to but included within thescope of the appended claims.

lVhat is claimed is:

1. A signalling system having substantially uniform transmission over a wide range of frequencies, including an inductive element the field of which contains a magnetic elementhaving an eddy current loss increasingwith frequency at signaling field strengths, the eddy current loss in said element being large at frequencies at which the other parts of the system tend to introduce a bend in the transmission characteristic whereby the tendency to the production of such bend in the characteristic is effectively opposed.

2, A signaling system having substantially uniform transmission over a wide range of frequencies, including an inductive element the field of which contains a magnetic element comprising an alloy consisting chiefly of nickel and iron and having an eddy current loss increasing with frequency more rapidly than iron at 'signaling'field strengths, said element being laminated, the eddy current loss in said element being large at fre qucncies at which the other parts of the system would introduce a bend in the transmissi on characteristic.

3. A wave transmission system comprising two circuits across one of which an appreciable capacity effect exists, means for coupling said circuits comprising inductance which tends to resonate with said capacity, and means associated with said inductance for controlling the frequency of said resonance comprising a magnetic structure the permeability of which decreases greatly in response to slight increases in the frequency of the transmitted waves.

4. A wave transmission system comprising two circuits across one of which an appreciable capacity effect exists, means for coupling said circuits comprising inductance which tends to resonate with said capacity, and means associated "withsaid inductance for controlling the frequency of said resonance comprising a magnetic structure lraving a higher initial permeability than silicon steel and proportioned to cause the value of the permeability to decrease greatlyzinrespouse to slight increases in the frequency of the transmitted waves.

5. A wave transmission system comprisin g two circuits across one'of which anappreciable capacity effect exists, a transformer for coupling said circuits and comprising inductance which tends to resonate with'said capacity, and a magnetic structure for said transformer comprising an alloy'consisting chiefly of iron and nickel having a higher permeabilitythan silicon steel at Zero frequency and proportioned to cause the value of the-permeability to decrease at predeter mined percentage in response to slight increases in the frequency of the transmitted waves.

6. A wave transmission system comprising two circuits across one" of which an appreciable capacity effect exists, 'atransformer for transmitting a band of frequencies comprised within the essential range of speech and music betweensaid circuits, said transformer comprising inductance which tends'ito resonate with said capacity, and means to cause the frequency of said resonance to occur at a point near the upper limit-of said band comprising a magnetic structure for said transformer which causes the-value of said inductance to decrease material-lyin response to increases in the frequency of the transmitted waves. 7. A transmission system comprising two circuits across one of which an appreciable capacity effect exists, a transformer for transmitting a wide band of frequencies between said circuits, said transformer comprisingmutual inductance which tends to resonate with said capacity, and means for varying the value of ,saidiinutual inductance within pre determined limits comprising a magnetic core for said transfori ner composed of a plurality of insulated lamin'ations of a material having a higher permeability at low magnetizing forces than silicon'steel, said laminations being proportioned in thickness to cause the eddy current losses in said core to vary in magnitude in response to changes'in the frequency of tl1e.trans1nitted waves such that the resonant frequency between said capacity and said mutual inductance is raised and the effective band width of the transmission characteristic is increased.

v 8. A-transmission system comprising twocircuits across one of which an appreciable capacity effect exists, a transformer for transmitting a wide band of frequencies between said circuits, said transformer com-prising mutual inductancezwhicl'i tends to resonate with said capacity, and means'for varying the value of said inductance within predetermined limits comprising a m-agnetic'core for said transformer composed of an alloy consisting chiefly of nickel and iron, in which the nickel contentis between 35% and 90% of the whole and is, so proportioned that the eddy current losses therein vary materially mag nitude in response to changes in the frequency ofthe transmitted waves.

9. In a transmission system comprising an incoming line and an out-going line, a circuit for repeating from said incoming line to said outgoing line electrical variations included within-a, wide frequency band, said circuit having a-capacityof the order of magnitude of the interelectrod'e capacity of a space dischargeamplifier, said. circuit including a transformer comprising inductance which tends to resonate with said capacity and the distributed capacity of said transformer at a frequency in the. lower region of said band, and a magnetic structure for said transformer the permeability of which decreases materially in response to increases in the frequency of said variations to raise the actual frequency of said resonance to a point well above the point attainable when the permeability of said magnetic structure remains constant.

10. In a transmission system comprising an incoming line and an outgoing line, a circuit for repeating from said incoming line to said outgoing line electrical variations extending over a wide frequency band, said circuit having a capacity of the order of magnitude of the interelectrode capacity of a space discharge amplifier, said circuit including a transformer comprising inductance which tends to resonate with said capacity and the distributed capacity of said transformer at a frequency in the lower region of said band, and a magnetic core for said transformer composed of an alloy consisting chiefly of nickel and iron in which the nickel content is 78 70 or thereabouts of the Whole and formed of a plurality of alminations of such cross-sectional area that the permeability of said core decreases materially in response to increases in the frequency of said variations, whereby. the actual frequency of said resonance is raised to a point in the neighborhood of the upper region of said band.

In witness whereof, I hereunto subscribe my name this 29 day of March, A. D. 1926.

FRANK E. FIELD. 

